Conventionally, a vibration actuator is known in which drive signals are applied to an electromechanical energy conversion element to thereby generate, in a vibrating body, driving vibrations in a plurality of bending modes in which the manner of bending is the same but the direction of bending is different, whereby a driven body brought into pressure or press contact with the vibrating body is frictionally driven (see e.g. PTL 1).
FIG. 14 is a perspective view of the vibration actuator 200. Further, FIG. 15 is an exploded perspective view of the vibration actuator 200 shown in FIG. 14. The vibration actuator 200 is comprised of a first elastic body 201, a piezoelectric unit 202, a flexible printed wiring board 203, a lower nut 204, a second elastic body 205, a shaft 206, a driven body 207, a gear 209, a coil spring 210, a fixing member 211, and an upper nut 212.
The first elastic body 201 is a disk-shaped member formed by a material, such as a metal, with a small vibration attenuation loss. The flexible printed wiring board 203 electrically connects a drive power source, not shown, and the piezoelectric unit 202. A drive signal is applied to a piezoelectric element as the electromechanical energy conversion element forming the piezoelectric unit 202 from the power source, not shown, via the flexible printed wiring board 203, whereby predetermined vibrations are generated in the piezoelectric unit 202.
The lower nut 204 is fitted on a screw portion formed in a lower end of the shaft 206. The shaft 206 is inserted into through holes formed in the respective central portions of the first elastic body 201, the piezoelectric unit 202, the flexible printed wiring board 203, and the second elastic body 205. Steps are provided at a generally central portion of the shaft 206 in a thrust direction thereof. Each of the steps is brought into abutment with an associated one of steps formed on an inner wall of the second elastic body 205. Further, a thread 231 is formed on an end of the shaft 206 toward the lower nut 204. The thread 231 is screwed into the lower nut 204 which is a fastening member, whereby the second elastic body 205, the first elastic body 201, the piezoelectric unit 202, and the flexible printed wiring board 203 are fastened and fixed by the shaft 206 and the lower nut 204 in the thrust direction of the shaft 206.
A contact spring portion having a spring property is formed at a lower part of the driven body 207. A surface of the first elastic body 201 on a side not in contact with the piezoelectric unit 202 is brought into pressure contact with the contact spring portion formed at the lower part of the driven body 207, whereby the driven body 207 receives a frictional driving force generated by the first elastic body 201. The gear 209 is an output unit for taking out a rotational output of the driven body 207 from the vibration actuator 200. The gear 209 is fitted on the driven body 207 such that the gear 209 permits the movement of the driven body 207 in the direction of a rotational axis, and rotates in unison with the driven body 207. The coil spring 210 as a pressure unit is disposed between a spring receiving portion of the driven body 207 and the gear 209, and urges the driven body 207 such that the driven body 207 is pushed down toward the first elastic body 201.
The gear 209 is supported by the fixing member 211 coupled to the shaft 206 in a manner rotatable about the shaft 206. Further, the position of the shaft 206 in the thrust direction is restricted by the fixing member 211. A thread 232 is formed also on an end of the shaft 206 toward the upper nut 212. The thread 232 is screwed into the upper nut 212, whereby the shaft 206 is fixed to the fixing member 211. The fixing member 211 is formed with screw holes. By attaching the fixing member 211 to a desired location of a desired member with screws, not shown, it is possible to attach the vibration actuator 200 to the desired location of the desired member.
FIG. 16 is an exploded perspective view of the piezoelectric unit 202. The piezoelectric unit 202 has a structure of a laminate of respective layers of a piezoelectric element 250_1 having the flexible printed wiring board 203 attached thereto, and a plurality of piezoelectric elements 250_2 to 250_n each having electrodes formed on one side thereof. The piezoelectric element 250_2 has driving electrodes A, A′, B and B′, and a vibration detection electrode S. The piezoelectric elements 250_3 to 250_n each have driving electrodes A, A′, B and B′ formed in a manner spaced from each other with a generally cross-shaped insulating portion therebetween. Each of the associated electrodes of the piezoelectric elements 250_1 to 250_n is electrically connected in a lamination direction e.g. by burying a conductive material in through holes formed in the piezoelectric elements 250_1 to 250, whereby each electrode is supplied with electric power via the through holes.
Note that in FIG. 16, the through holes and the electrodes electrically connected in the lamination direction are denoted by the same reference numerals (A, A′, B and B′). For example, the through holes A of the piezoelectric element 250_3 are electrically connected to the respective associated electrodes A of the piezoelectric element 250_4. Further, the electrodes of the piezoelectric elements 250_3 to 250_n are each formed as four areas spaced from each other with the generally cross-shaped insulating portion therebetween so as to make effective use of the driving force of the vibration actuator 200, but detailed description thereof is omitted. Piezoelectric bodies of the piezoelectric elements 250_2 to 250_n−1 are each oppositely polarized in the lamination direction on a layer-by-layer basis.
Drive signals different in phase are applied from the flexible printed wiring board 203 to the electrodes A, A′, B and B′ via the through holes formed in the piezoelectric element 250_1, whereby driving by the vibration actuator 200 is realized. The electrodes A and B as well as the electrodes A′ and B′ are shifted from each other by 90° in position phase. The drive signals applied to the respective electrodes A′ and B′ are shifted from the drive signals applied to the respective electrodes A and B by 180° in time phase, respectively. For example, when the drive signals shifted from each other by 180° in time phase are applied to the electrodes A and A′, areas of the electrodes A of each piezoelectric body expand in a direction of the thickness thereof, but areas of the electrodes A′ of each piezoelectric body contract in the direction of the thickness thereof. As a consequence, a bending vibration in which the piezoelectric unit 202 bends around the shaft 206 in an electrode A-electrode A′ direction is generated, thereby shaking the first elastic body 201 in the electrode A-electrode A′ direction.
Here, if the drive signals shifted from the respective drive signals applied to the electrodes A and A′ by 90° in time phase are applied to the electrodes B and B′, respectively, two bending vibrations, i.e. the above-mentioned bending vibration which shakes the first elastic body 201 in the electrode A-electrode A′ direction, and a bending vibration which shakes the first elastic body 201 in an electrode B-electrode B′ direction, are generated in the piezoelectric unit 202. These vibrations are combined to thereby excite progressive elliptic motions in the surface of the first elastic body 201. In the vibration actuator 200, the driven body 207 (the contact spring portion thereof) is brought into pressure contact with the surface of the first elastic body 201, in which the elliptic motion has been excited, and therefore the driven body 207 is moved (rotated) in a manner pushed forward by the elliptic motion of the first elastic body 201.
For example, in a case where a focus lens of an image pickup apparatus is driven using the vibration actuator 200, the vibration actuator 200 is required to be adapted to the motion of the focus lens for focusing on an object at a high speed and the motion of the same for continuing to focus on an object the position of which is being shifted slowly. To meet this requirement, the vibration actuator 200 is required to be capable of operating in a speed range of ten or more times in terms of a speed ratio between high and low speeds. Particularly, there is an increasing need for stably driving the focus lens at a low speed. To meet such requirements, as a method of driving the vibration actuator 200 at a low speed, there has been proposed a method in which the phase differences between drive signals applied to the electrodes A, A′, B, and B′ are changed to reduce a vibration amplitude in a direction of driving the focus lens.